This invention relates to an apparatus for measuring the proportion or quantity of a component in a radiation-transparent mixture by means of discrete radiation absorption. The beam passing through the mixture is split into a measuring beam and a reference beam. The measuring beam is passed through a narrow-band interference filter, the transmitting wavelength of which corresponds to a discrete absorption wavelength of the component to be tested. The reference beam is passed through a narrow-band interference filter, the transmitting wavelength of which is different from the absorption wavelength of the component to be tested. The difference between the radiation intensities of measuring beam and reference beam is utilized as the measure for the proportion or quantity, as the case may be, of the component to be tested.
A measuring method which is based on infra-red absorption and which relates to the determination of individual components of a liquid or gaseous mixture, is described in J. J. Howarth et al., An Infra-red Process Analyser Based on Interference Filters, JOURNAL OF SCIENTIFIC INSTRUMENTS, Volume 42, 1965, pages 526-528. According to the method described in this article, two branch beams are passed through a measuring filter and a reference filter and subsequently they are directed by means of appropriate deflecting systems to a common detector. Between the detector and the filters there is arranged a chopper which alternately transmits the measuring beam and the reference beam. As set forth in the first paragraph in the righthand column on page 528 of the above-identified article, by virtue of an appropriate filter combination an indicating sensitivity to hydrogen fluoride down to 50 parts per million was obtained. In the same paragraph of the article it is indicated that the interference bands and the absorption bands have to match exactly.
Particularly in the use of narrow-band interference filters it has been found to be difficult to adjust the sensitivity maximum in a permanent manner by an appropriate setting of the filters. Because of the steep flanks of the absorption lines of the material and the transmission lines of the filters, slight external influences, such as small mechanical vibrations and the like, suffice to lower the sensitivity and thus the accuracy of value indication.